The invention relates to a system for preparing a de-icing mixture of glycol and water for use on demand in the spraying of aircraft prior to take-off under icing conditions.
It has been a general practice to prepare de-icing mixture of the character on a motorized truck and just prior to discharge upon wing and fuselage surfaces of an involved aircraft. The truck has its own tanks for the separate containment of glycol and water, as sources for pumped extraction, mixing, and heating prior to discharge. The ability to control temperature and mixing proportions is poor, and because safety of aircraft is primary, excessively rich discharges are the rule. Excessive richness is wasteful of glycol, and the rising cost of glycol is making such wasteful use intolerable. U.S. Pat. No. 4,723,733 discloses a truck of the character indicated, and German Auslegeschrift No. 1,266,139 also discloses truck-borne mixing apparatus.
It has been another practice to prepare, accumulate and maintain a reservoir of mixed and heated de-icing fluid, i.e., in bulk, in readiness for use when required. But this technique has various disadvantages, not the least of which is that continuous maintenance of desired temperature, e.g., 180.degree. F., entails (1) the expense of continuous heating, (2) the progressive enrichment of the mixed fluid as water is vaporized, and (3) an inability to correct the mixed proportions in response to a change in ambient temperature and/or wind conditions. U.S. Pat. Nos. 3,533,395 and 4,191,348 disclose bulk-preparation systems of this general nature, and subject to the indicated disadvantages.
Published PCT Application WO 79/00331 and U.S. Pat. No. 4,634,084 disclose overhead spraying structure through which an aircraft is moved in the course of a spraying cycle. The problem of glycol waste and environmental protection is recognized by a system of underground waste-collection piping, fed by spilled-mixture seepage through a porous pavement. The underground-collected mixture is returned to a sump, for accumulation, reprocessing and recycled use via the spraying system. These patents are concerned either with the control of spraying for different aircraft configurations, or with detail of the porous pavement; but these patents are uninformative in the matter of how to achieve economies in the mixing and supply of de-icing fluid.
U.S. Pat. No. 3,612,075 also discloses an overhead system for spraying aircraft of different configurations. There is recognition of problems arising from extreme weather conditions (i.e., high winds, heavy snow), but the patent is silent on any satisfactory means for control, other than manual control, in the face of such conditions.
Published PCT Application WO 86/00592 discloses a computer-operated system which responds to ambient conditions of temperature, including wing-surface temperature, and which controls the mixing of water and glycol in accordance with a program-selected mixture ratio. Mixture ratio is controlled by automated control of the respective rates of flow of water and glycol; these rates are individually measured at all times to determine the instantaneous or "real" flow rates, and from these measurements of "real" flow rates, the "real" mixture ratio is calculated. The calculated "real" mixture ratio is available as an electric feedback signal for comparison with a signal representing the program-selected mixture ratio, and the difference between these signals is used to effect appropriate correction of the respective flow rates. This system is subject to several deficiencies, not the least of which is the fact that measurement of flow rates alone will not provide a proper index of the de-icing capability of the mixed fluid, in that the mixture ratio is determined from comparison of flow rates, on the unrealistic assumption that the glycol flow is always of the same quality, whereas it may be and often is subject to an unknown fractional content of water dilution.